Not Applicable.
Not Applicable.
The present invention relates generally to imaging systems, and more particularly, to imaging systems for imaging physiological functions.
Measuring blood flow within the body can be a useful tool in diagnosing and treating patients. As known to one of ordinary skill in the art, certain organs in the body, such as the brain and heart, are damaged relatively quickly without an adequate flow of blood. The amount of blood flow can be an important factor in determining the optimal treatment option for a patient.
There are a variety of known systems for obtaining blood flow information from various locations in the body, such as the brain. One technique for obtaining quantitative blood flow information is Positron Emission Tomography (PET). PET is not widely used due to several practical and medical disadvantages. For example, PET systems are relatively expensive to operate and require the use of a cyclotron, which is not generally available. In addition, PET requires the use of radionucleotides, which are potentially harmful to a patient. Furthermore, the anatomical resolution of PET is limited, i.e., significantly less than Magnetic Resonance Imaging (MRI).
Another technique for measuring blood flow is known as Xenon enhanced CT (computed tomography). Like PET, Xenon-enhanced CT can be uncomfortable to the patient, exposes the patient to ionizing radiation, and is limited in anatomical resolution.
Another known technique for obtaining blood flow information is known as perfusion weighted Magnetic Resonance Imaging (MRI) or PWI. In general, MRI systems provide a relatively high degree of anatomical resolution. A common type of PWI relies upon the temporal characteristics of a paramagnetic chelate, such as a gadolinium derivative, delivered as a bolus intravascularly. The chelate functions as a contrast agent for monitoring the signal intensity of the vasculature. In general, the signal intensity decreases relative to the surrounding tissue, which serves as the basis to image the tissue. If the vasculature is intact in the region of interest, e.g., for the brain there is no leakiness in the blood-brain barrier, the signal drop can be used to image the blood flow in the brain. Image analysis over time can be used to determine relative differences in blood volume, flow, and mean transit time.
While PWI can be used to compare relative blood flows at different locations, such as on left and right sides of the brain, this information may be of limited utility. For example, PWI can identify a problem in the case where one side of the brain has one half the blood flow of the other side of the brain. However, if both sides of the brain have half the normal blood flow this reduction in flow may not be identified.
FIG. 1 shows an artery 2 providing blood to a region of tissue, such as a capillary bed 4, within an organ 6. In a common form of PWI, the natural logarithm of the signal change in the tissue is estimated to be proportional to the concentration of the MRI contrast agent, when a T2 contrast agent and appropriate MRI parameters are used. (A different mathematical relationship between contrast agent concentration and MRI signal change is present with other types of contrast agents such as T1-based agents.) However, the signal change may not be proportional to MRI contrast agent concentration in larger blood vessels. Thus, an MRI-derived arterial input function (AIF) provides limited ability to determine quantitative blood perfusion indices. That is, PWI is generally limited to providing relative perfusion information due to the nonlinear nature of the signal change in relatively large blood vessels.
It would, therefore, be desirable to provide a minimally invasive technique for determining quantitative perfusion indices with relatively high anatomical resolution without the injection of harmful agents or the use of ionizing radiation.
The present invention provides a method for determining quantitative perfusion indices using magnetic resonance imaging (MRI) and optical densitometry. In general, a bolus containing an MRI contrast agent (tracer) and an optical contrast agent is injected into a patient. MRI is used to determine tracer concentration in a tissue volume of interest (VOI) and optical densitometry is used to determine the arterial input function. Using deconvolution, quantitative blood flow information can be determined.
In one aspect of the invention, a bolus containing an optical contrast agent and an MRI contrast agent is administered to a patient. Optical contrast agent concentration is sampled over time, such as by optical densitometry or fluoresence to derive an arterial input function. The concentration-time curve of the MRI contrast agent is determined using MRI to derive a tissue function. The optical and MRI contrast agents are sampled at known times such that the samples can be correlated in time. From the known relationship between the MRI and optical contrast agents in the bolus, quantitative blood flow information can be derived using deconvolution.
In a further aspect of the invention, a solution containing an optical contrast agent and an MRI contrast agent is provided. In one embodiment, the optical contrast agent includes Indocyanine Green (ICG) and the MRI contrast agent includes gadopentate dimegluminepentaacetic acid (Gd-DTPA), also known by the trade name Magnevist. The solution can be used as a bolus injection to determine quantitative blood flow information from optical and MRI time-concentration data.